1. Field of the Invention
The present invention relates to semiconductor devices and, in particular, to starter current source devices for use with analog circuits and processes for their manufacture.
2. Description of the Related Art
A variety of Metal-Oxide-Semiconductor (MOS) and bipolar analog device circuits, including current mirrors, self-biased circuits, voltage references and current references, require a start-up device (e.g. a starter current source device) to shift such circuits from an undesired equilibrium state at the commencement of their operation. See, for example, Phillip E. Allen and Douglas R. Holberg, Current and Voltage References, in CMOS Analog Circuit Design, 240-251 (Oxford University Press, 1987) and Paul R. Gray and Robert G. Meyer, Transistor Current Sources and Active Leads, in Analysis and Design of Analog Integrated Circuits, 326-331 (John Wiley & Sons, Inc., 1993), both of which are hereby incorporated by reference.
An example of an analog device circuit that requires a start-up device is the current mirror 10 shown schematically in FIG. 1. Current mirror 10 includes interconnected MOS transistors 11, 12, 13 and 14 and can have, on commencement of its operation, its node A at ground potential and its node B at V.sub.dd, therefore being at an equilibrium state. In this state, both the left-hand branch (i.e. MOS transistors 11 and 13) and right-hand branch (i.e. MOS transistors 12 and 14) of the current mirror 10 are shut-down. This undesirable equilibrium state can be prevented by providing a separate starter current source device 16 connected to node B of the current mirror, as illustrated in FIG. 1.
Conventional starter current source devices for use with current mirrors can take the form of: (i) a resistor 20 connected between node B of the current mirror and ground (GND), as shown in FIG. 2; (ii) an MOS transistor 30 with its gate connected to V.sub.dd, while its source is connected to node B of the current mirror and its drain is connected to ground, as shown in FIG. 3; and (iii) an MOS transistor 40 with its gate connected to a reference voltage (V.sub.ref), while its source is connected to node B of the current mirror and its drain is connected to ground, as shown in FIG. 4. These starter current source devices are used to supply a sufficient amount of current to shift the potential at node B of the current mirror 10 closer to ground potential, thereby allowing current to flow through the right-hand branch MOS transistor 12 connected between V.sub.dd and node B. An identical amount of current, as is flowing through the right-hand branch, then also flows in the left-hand branch of the current mirror 10, thereby raising the potential of node A towards V.sub.dd and ensuring the proper commencement of operation of current mirror 10.
A drawback of conventional starter current source devices is that current continues to flow through the devices and power consumption persists, even after the associated analog device circuit has commenced its desired operation.
Still needed in the art is a starter current source device that is capable of being shut-down automatically upon commencement of operation of an associated analog circuit. Also needed is a process for its manufacture that is simple and compatible with standard semiconductor device processing.